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  <div class="section" id="functionality-overview">
<span id="id1"></span><h1>3. Functionality overview</h1>
<p>This section provides a high-level overview of the functionality provided by the
interface defined in this specification. Refer to the
<a class="reference internal" href="../api/library/index.html#api-reference"><span class="std std-ref">API definition</span></a> for a detailed description.</p>
<p><a class="reference internal" href="../appendix/history.html#future"><span class="std std-ref">Future additions</span></a> describes features that might be included in future versions of this
specification.</p>
<p>Due to the modularity of the interface, almost every part of the library is
optional. The only mandatory function is <a class="reference internal" href="../api/library/library.html#c.psa_crypto_init" title="psa_crypto_init"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_crypto_init()</span></code></a>.</p>
<div class="section" id="library-management">
<h2>3.1. Library management</h2>
<p>Applications must call <a class="reference internal" href="../api/library/library.html#c.psa_crypto_init" title="psa_crypto_init"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_crypto_init()</span></code></a> to initialize the library before
using any other function.</p>
</div>
<div class="section" id="key-management">
<h2>3.2. Key management</h2>
<p>Applications always access keys indirectly via an identifier, and can perform
operations using a key without accessing the key material. This allows keys to
be <em>non-extractable</em>, where an application can use a key but is not permitted to
obtain the key material. Non-extractable keys are bound to the device, can be
rate-limited and can have their usage restricted by policies.</p>
<p>Each key has a set of attributes that describe the key and the policy for using
the key. A <a class="reference internal" href="../api/keys/attributes.html#c.psa_key_attributes_t" title="psa_key_attributes_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_attributes_t</span></code></a> object contains all of the attributes, which
is used when creating a key and when querying key attributes.</p>
<p>The key attributes include:</p>
<ul class="simple">
<li><p>A <a class="reference internal" href="../api/keys/types.html#key-types"><span class="std std-ref">type</span></a> and size that describe the key material.</p></li>
<li><p>The key <a class="reference internal" href="#key-ids"><span class="std std-ref">identifier</span></a> that the application uses to refer to the key.</p></li>
<li><p>A <a class="reference internal" href="#key-life"><span class="std std-ref">lifetime</span></a> that determines when the key material is destroyed, and where it is stored.</p></li>
<li><p>A <a class="reference internal" href="#key-usage-policies"><span class="std std-ref">policy</span></a> that determines how the key can be used.</p></li>
</ul>
<p>Keys are created using one of the <em>key creation functions</em>:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_import_key" title="psa_import_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_import_key()</span></code></a></p></li>
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_generate_key" title="psa_generate_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_generate_key()</span></code></a></p></li>
<li><p><a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_output_key" title="psa_key_derivation_output_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_output_key()</span></code></a></p></li>
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_copy_key" title="psa_copy_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_copy_key()</span></code></a></p></li>
</ul>
<p>These output the key identifier, that is used to access the key in all other parts of the API.</p>
<p>All of the key attributes are set when the key is created and cannot be changed
without destroying the key first. If the original key permits copying, then the
application can specify a different lifetime or restricted policy for the
copy of the key.</p>
<p>A call to <a class="reference internal" href="../api/keys/management.html#c.psa_destroy_key" title="psa_destroy_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_destroy_key()</span></code></a> destroys the key material, and will cause any active
operations that are using the key to fail. Therefore an application must not
destroy a key while an operation using that key is in progress, unless the
application is prepared to handle a failure of the operation.</p>
<div class="section" id="key-identifiers">
<span id="key-ids"></span><h3>3.2.1. Key identifiers</h3>
<p>Key identifiers are integral values that act as permanent names for persistent keys, or as transient references to volatile keys. Key identifiers are defined by the application for persistent keys, and by the implementation for volatile keys and for built-in keys.</p>
<p>Key identifiers are output from a successful call to one of the key creation functions.</p>
<p>Valid key identifiers must have distinct values within the same application. If
the implementation provides <a class="reference internal" href="goals.html#isolation"><span class="std std-ref">caller isolation</span></a>, then key
identifiers are local to each application. That is, the same key identifier in two
applications corresponds to two different keys.</p>
<p>See <a class="reference internal" href="../api/keys/ids.html#key-identifiers"><span class="secref">Key identifiers</span></a>.</p>
</div>
<div class="section" id="key-lifetimes">
<span id="key-life"></span><h3>3.2.2. Key lifetimes</h3>
<p>The lifetime of a key indicates where it is stored and which application and system actions will create and destroy it.</p>
<p>There are two main types of lifetimes: <em>volatile</em> and <em>persistent</em>.</p>
<p>Volatile keys are automatically destroyed when the application instance terminates or on a power reset of the device. Volatile key identifiers are allocated by the implementation when the key is created. Volatile keys can be explicitly destroyed with a call to <a class="reference internal" href="../api/keys/management.html#c.psa_destroy_key" title="psa_destroy_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_destroy_key()</span></code></a>.</p>
<p>Persistent keys are preserved until the application explicitly destroys them or until an implementation-specific device management event occurs, for example, a factory reset. The key identifier for a persistent key is set by the application when creating the key, and remains valid throughout the lifetime of the key, even if the application instance that created the key terminates.</p>
<p>See <a class="reference internal" href="../api/keys/lifetimes.html#key-lifetimes"><span class="secref">Key lifetimes</span></a>.</p>
</div>
<div class="section" id="key-policies">
<span id="key-usage-policies"></span><h3>3.2.3. Key policies</h3>
<p>All keys have an associated policy that regulates which operations are permitted on the key. Each key policy is a set of usage flags and a specific algorithm that is permitted with the key. See <a class="reference internal" href="../api/keys/policy.html#key-policy"><span class="secref">Key policies</span></a>.</p>
</div>
<div class="section" id="recommendations-of-minimum-standards-for-key-management">
<h3>3.2.4. Recommendations of minimum standards for key management</h3>
<p>Most implementations provide the following functions:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_import_key" title="psa_import_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_import_key()</span></code></a>. The exceptions are implementations that only give access
to a key or keys that are provisioned by proprietary means, and do not allow
the main application to use its own cryptographic material.</p></li>
<li><p><a class="reference internal" href="../api/keys/attributes.html#c.psa_get_key_attributes" title="psa_get_key_attributes"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_get_key_attributes()</span></code></a> and the <code class="docutils literal notranslate"><span class="pre">psa_get_key_xxx()</span></code> accessor functions.
They are easy to implement, and it is difficult to write applications and to
diagnose issues without being able to check the metadata.</p></li>
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_export_public_key" title="psa_export_public_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_export_public_key()</span></code></a>. This function is usually provided if the
implementation supports any asymmetric algorithm, since public-key
cryptography often requires the delivery of a public key that is associated
with a protected private key.</p></li>
<li><p><a class="reference internal" href="../api/keys/management.html#c.psa_export_key" title="psa_export_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_export_key()</span></code></a>. However, highly constrained implementations that are
designed to work only with short-term keys, or only with long-term
non-extractable keys, do not need to provide this function.</p></li>
</ul>
</div>
</div>
<div class="section" id="symmetric-cryptography">
<h2>3.3. Symmetric cryptography</h2>
<p>This specification defines interfaces for the following types of symmetric
cryptographic operation:</p>
<ul class="simple">
<li><p>Message digests, commonly known as hash functions.</p></li>
<li><p>Message authentication codes (MAC).</p></li>
<li><p>Symmetric ciphers.</p></li>
<li><p>Authenticated encryption with associated data (AEAD).</p></li>
</ul>
<p>For each type of symmetric cryptographic operation, the API includes:</p>
<ul class="simple">
<li><p>A pair of <em>single-part</em> functions. For example, compute and verify, or
encrypt and decrypt.</p></li>
<li><p>A series of functions that permit <em>multi-part operations</em>.</p></li>
</ul>
<div class="section" id="single-part-functions">
<h3>3.3.1. Single-part Functions</h3>
<p>Single-part functions are APIs that implement the cryptographic operation in a
single function call. This is the easiest API to use when all of the inputs and
outputs fit into the application memory.</p>
<p>Some use cases involve messages that are too large to be assembled in memory, or
require non-default configuration of the algorithm. These use cases require the
use of a <a class="reference internal" href="#multi-part-operations"><span class="secref">multi-part operation</span></a>.</p>
</div>
<div class="section" id="multi-part-operations">
<span id="id2"></span><h3>3.3.2. Multi-part operations</h3>
<p>Multi-part operations are APIs which split a single cryptographic operation into
a sequence of separate steps. This enables fine control over the configuration
of the cryptographic operation, and allows the message data to be processed in
fragments instead of all at once. For example, the following situations require
the use of a multi-part operation:</p>
<ul class="simple">
<li><p>Processing messages that cannot be assembled in memory.</p></li>
<li><p>Using a deterministic IV for unauthenticated encryption.</p></li>
<li><p>Providing the IV separately for unauthenticated encryption or decryption.</p></li>
<li><p>Separating the AEAD authentication tag from the cipher text.</p></li>
</ul>
<p>Each multi-part operation defines a specific object type to maintain the state
of the operation. These types are implementation-defined. All multi-part
operations follow the same pattern of use:</p>
<ol class="arabic">
<li><p><strong>Allocate:</strong> Allocate memory for an operation object of the appropriate
type. The application can use any allocation strategy: stack, heap, static, etc.</p></li>
<li><p><strong>Initialize:</strong> Initialize or assign the operation object by one of the
following methods:</p>
<blockquote>
<div><ul class="simple">
<li><p>Set it to logical zero. This is automatic for static and global
variables. Explicit initialization must use the associated
<code class="docutils literal notranslate"><span class="pre">PSA_xxx_INIT</span></code> macro as the type is implementation-defined.</p></li>
<li><p>Set it to all-bits zero. This is automatic if the object was
allocated with <code class="docutils literal notranslate"><span class="pre">calloc()</span></code>.</p></li>
<li><p>Assign the value of the associated macro <code class="docutils literal notranslate"><span class="pre">PSA_xxx_INIT</span></code>.</p></li>
<li><p>Assign the result of calling the associated function
<code class="docutils literal notranslate"><span class="pre">psa_xxx_init()</span></code>.</p></li>
</ul>
</div></blockquote>
<p>The resulting object is now <em>inactive</em>.</p>
<p>It is an error to initialize an operation object that is in <em>active</em> or
<em>error</em> states. This can leak memory or other resources.</p>
</li>
<li><p><strong>Setup:</strong> Start a new multi-part operation on an <em>inactive</em> operation
object. Each operation object will define one or more setup functions to
start a specific operation.</p>
<p>On success, a setup function will put an operation object into an <em>active</em>
state. On failure, the operation object will remain <em>inactive</em>.</p>
</li>
<li><p><strong>Update:</strong> Update an <em>active</em> operation object. The update function can
provide additional parameters, supply data for processing or generate
outputs.</p>
<p>On success, the operation object remains <em>active</em>. On failure, the
operation object will enter an <em>error</em> state.</p>
</li>
<li><p><strong>Finish:</strong> To end the operation, call the applicable finishing function.
This will take any final inputs, produce any final outputs, and then
release any resources associated with the operation.</p>
<p>On success, the operation object returns to the <em>inactive</em> state. On
failure, the operation object will enter an <em>error</em> state.</p>
</li>
</ol>
<p>An operation can be aborted at any stage during its use by calling the
associated <code class="docutils literal notranslate"><span class="pre">psa_xxx_abort()</span></code> function. This will release any resources
associated with the operation and return the operation object to the <em>inactive</em>
state.</p>
<p>Any error that occurs to an operation while it is in an <em>active</em> state will
result in the operation entering an <em>error</em> state. The application must call the
associated <code class="docutils literal notranslate"><span class="pre">psa_xxx_abort()</span></code> function to release the operation resources and
return the object to the <em>inactive</em> state.</p>
<p>Once an operation object is returned to the <em>inactive</em> state, it can be reused
by calling one of the applicable setup functions again.</p>
<p>If a multi-part operation object is not initialized before use, the behavior is
undefined.</p>
<p>If a multi-part operation function determines that the operation object is not in
any valid state, it can return <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_CORRUPTION_DETECTED" title="PSA_ERROR_CORRUPTION_DETECTED"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_CORRUPTION_DETECTED</span></code></a>.</p>
<p>If a multi-part operation function is called with an operation object in the
wrong state, the function will return <a class="reference internal" href="../api/library/status.html#c.PSA_ERROR_BAD_STATE" title="PSA_ERROR_BAD_STATE"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ERROR_BAD_STATE</span></code></a> and the operation
object will enter the <em>error</em> state.</p>
<p>It is safe to move a multi-part operation object to a different memory location,
for example, using a bitwise copy, and then to use the object in the new
location. For example, an application can allocate an operation object on the
stack and return it, or the operation object can be allocated within memory
managed by a garbage collector. However, this does not permit the following
behaviors:</p>
<ul class="simple">
<li><p>Moving the object while a function is being called on the object. This is
not safe. See also <a class="reference internal" href="conventions.html#concurrency"><span class="std std-ref">Concurrent calls</span></a>.</p></li>
<li><p>Working with both the original and the copied operation objects. This
requires cloning the operation, which is only available for hash operations
using <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_clone" title="psa_hash_clone"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_clone()</span></code></a>.</p></li>
</ul>
<p>Each type of multi-part operation can have multiple <em>active</em> states.
Documentation for the specific operation describes the configuration and update
functions, and any requirements about their usage and ordering.</p>
</div>
<div class="section" id="message-digests-hashes">
<h3>3.3.3. Message digests (Hashes)</h3>
<p>The single-part hash functions are:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_compute" title="psa_hash_compute"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_compute()</span></code></a> to calculate the hash of a message.</p></li>
<li><p><a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_compare" title="psa_hash_compare"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_compare()</span></code></a> to compare the hash of a message with a reference value.</p></li>
</ul>
<p>The <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_operation_t" title="psa_hash_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_hash_operation_t</span></code></a> <a class="reference internal" href="#multi-part-operations"><span class="std std-ref">multi-part operation</span></a>
allows messages to be processed in fragments:</p>
<ol class="arabic simple">
<li><p>Initialize the <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_operation_t" title="psa_hash_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_hash_operation_t</span></code></a> object to zero, or by assigning the
value of the associated macro <a class="reference internal" href="../api/ops/hashes.html#c.PSA_HASH_OPERATION_INIT" title="PSA_HASH_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_HASH_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_setup" title="psa_hash_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_setup()</span></code></a> to specify the required hash algorithm, call
<a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_clone" title="psa_hash_clone"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_clone()</span></code></a> to duplicate the state of <em>active</em> <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_operation_t" title="psa_hash_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_hash_operation_t</span></code></a>
object, or call <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_resume" title="psa_hash_resume"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_resume()</span></code></a> to restart a hash operation with the
output from a previously suspended hash operation.</p></li>
<li><p>Call the <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_update" title="psa_hash_update"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_update()</span></code></a> function on successive chunks of the message.</p></li>
<li><p>At the end of the message, call the required finishing function:</p>
<ul class="simple">
<li><p>To suspend the hash operation and extract a hash suspend state,
call <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_suspend" title="psa_hash_suspend"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_suspend()</span></code></a>. The output state can subsequently be used
to resume the hash operation.</p></li>
<li><p>To calculate the digest of a message, call <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_finish" title="psa_hash_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_finish()</span></code></a>.</p></li>
<li><p>To verify the digest of a message against a reference value, call
<a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_verify" title="psa_hash_verify"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_verify()</span></code></a>.</p></li>
</ul>
</li>
</ol>
<p>To abort the operation or recover from an error, call <a class="reference internal" href="../api/ops/hashes.html#c.psa_hash_abort" title="psa_hash_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_hash_abort()</span></code></a>.</p>
</div>
<div class="section" id="message-authentication-codes-macs">
<h3>3.3.4. Message authentication codes (MACs)</h3>
<p>The single-part MAC functions are:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/ops/macs.html#c.psa_mac_compute" title="psa_mac_compute"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_compute()</span></code></a> to calculate the MAC of a message.</p></li>
<li><p><a class="reference internal" href="../api/ops/macs.html#c.psa_mac_verify" title="psa_mac_verify"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_verify()</span></code></a> to compare the MAC of a message with a reference value.</p></li>
</ul>
<p>The <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_operation_t" title="psa_mac_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_mac_operation_t</span></code></a> <a class="reference internal" href="#multi-part-operations"><span class="std std-ref">multi-part operation</span></a>
allows messages to be processed in fragments:</p>
<ol class="arabic simple">
<li><p>Initialize the <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_operation_t" title="psa_mac_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_mac_operation_t</span></code></a> object to zero, or by assigning the
value of the associated macro <a class="reference internal" href="../api/ops/macs.html#c.PSA_MAC_OPERATION_INIT" title="PSA_MAC_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_MAC_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_sign_setup" title="psa_mac_sign_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_sign_setup()</span></code></a> or <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_verify_setup" title="psa_mac_verify_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_verify_setup()</span></code></a> to specify the
algorithm and key.</p></li>
<li><p>Call the <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_update" title="psa_mac_update"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_update()</span></code></a> function on successive chunks of the message.</p></li>
<li><p>At the end of the message, call the required finishing function:</p>
<ul class="simple">
<li><p>To calculate the MAC of the message, call <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_sign_finish" title="psa_mac_sign_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_sign_finish()</span></code></a>.</p></li>
<li><p>To verify the MAC of the message against a reference value, call
<a class="reference internal" href="../api/ops/macs.html#c.psa_mac_verify_finish" title="psa_mac_verify_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_verify_finish()</span></code></a>.</p></li>
</ul>
</li>
</ol>
<p>To abort the operation or recover from an error, call <a class="reference internal" href="../api/ops/macs.html#c.psa_mac_abort" title="psa_mac_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_mac_abort()</span></code></a>.</p>
</div>
<div class="section" id="encryption-and-decryption">
<h3>3.3.5. Encryption and decryption</h3>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The unauthenticated cipher API is provided to implement legacy protocols and
for use cases where the data integrity and authenticity is guaranteed by
non-cryptographic means. It is recommended that newer protocols use
<a class="reference internal" href="#func-aead"><span class="secref">Authenticated encryption (AEAD)</span></a>.</p>
</div>
<p>The single-part functions for encrypting or decrypting a message using an
unauthenticated symmetric cipher are:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_encrypt" title="psa_cipher_encrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_encrypt()</span></code></a> to encrypt a message using an unauthenticated symmetric
cipher. The encryption function generates a random IV. Use the multi-part API
to provide a deterministic IV: this is not secure in general, but
can be secure in some conditions that depend on the algorithm.</p></li>
<li><p><a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_decrypt" title="psa_cipher_decrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_decrypt()</span></code></a> to decrypt a message using an unauthenticated symmetric
cipher.</p></li>
</ul>
<p>The <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_operation_t" title="psa_cipher_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_cipher_operation_t</span></code></a> <a class="reference internal" href="#multi-part-operations"><span class="std std-ref">multi-part operation</span></a>
permits alternative initialization parameters and allows messages to be
processed in fragments:</p>
<ol class="arabic simple">
<li><p>Initialize the <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_operation_t" title="psa_cipher_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_cipher_operation_t</span></code></a> object to zero, or by assigning the
value of the associated macro <a class="reference internal" href="../api/ops/ciphers.html#c.PSA_CIPHER_OPERATION_INIT" title="PSA_CIPHER_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_CIPHER_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_encrypt_setup" title="psa_cipher_encrypt_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_encrypt_setup()</span></code></a> or <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_decrypt_setup" title="psa_cipher_decrypt_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_decrypt_setup()</span></code></a> to specify the
algorithm and key.</p></li>
<li><p>Provide additional parameters:</p>
<ul class="simple">
<li><p>When encrypting data, generate or set an initialization vector (IV),
nonce, or similar initial value such as an initial counter value. To
generate a random IV, which is recommended in most protocols, call
<a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_generate_iv" title="psa_cipher_generate_iv"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_generate_iv()</span></code></a>. To set the IV, call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_set_iv" title="psa_cipher_set_iv"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_set_iv()</span></code></a>.</p></li>
<li><p>When decrypting, set the IV or nonce. To set the IV, call
<a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_set_iv" title="psa_cipher_set_iv"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_set_iv()</span></code></a>.</p></li>
</ul>
</li>
<li><p>Call the <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_update" title="psa_cipher_update"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_update()</span></code></a> function on successive chunks of the message.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_finish" title="psa_cipher_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_finish()</span></code></a> to complete the operation and return any final
output.</p></li>
</ol>
<p>To abort the operation or recover from an error, call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_abort" title="psa_cipher_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_abort()</span></code></a>.</p>
</div>
<div class="section" id="authenticated-encryption-aead">
<span id="func-aead"></span><h3>3.3.6. Authenticated encryption (AEAD)</h3>
<p>The single-part AEAD functions are:</p>
<ul class="simple">
<li><p><a class="reference internal" href="../api/ops/aead.html#c.psa_aead_encrypt" title="psa_aead_encrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_encrypt()</span></code></a> to encrypt a message using an authenticated symmetric
cipher.</p></li>
<li><p><a class="reference internal" href="../api/ops/aead.html#c.psa_aead_decrypt" title="psa_aead_decrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_decrypt()</span></code></a> to decrypt a message using an authenticated symmetric
cipher.</p></li>
</ul>
<p>These functions follow the interface recommended by <span><em>An Interface and Algorithms for Authenticated Encryption</em> <a class="reference internal" href="../about.html#citation-rfc5116"><span class="cite">[RFC5116]</span></a></span>.</p>
<p>The encryption function requires a nonce to be provided. To generate a random
nonce, either call <a class="reference internal" href="../api/ops/rng.html#c.psa_generate_random" title="psa_generate_random"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_generate_random()</span></code></a> or use the AEAD multi-part API.</p>
<p>The <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_operation_t" title="psa_aead_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_aead_operation_t</span></code></a> <a class="reference internal" href="#multi-part-operations"><span class="std std-ref">multi-part operation</span></a>
permits alternative initialization parameters and allows messages to be
processed in fragments:</p>
<ol class="arabic simple">
<li><p>Initialize the <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_operation_t" title="psa_aead_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_aead_operation_t</span></code></a> object to zero, or by assigning the
value of the associated macro <a class="reference internal" href="../api/ops/aead.html#c.PSA_AEAD_OPERATION_INIT" title="PSA_AEAD_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_AEAD_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_encrypt_setup" title="psa_aead_encrypt_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_encrypt_setup()</span></code></a> or <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_decrypt_setup" title="psa_aead_decrypt_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_decrypt_setup()</span></code></a> to specify the
algorithm and key.</p></li>
<li><p>Provide additional parameters:</p>
<ul class="simple">
<li><p>If the algorithm requires it, call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_set_lengths" title="psa_aead_set_lengths"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_set_lengths()</span></code></a> to specify the
length of the non-encrypted and encrypted inputs to the operation.</p></li>
<li><p>When encrypting, call either <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_generate_nonce" title="psa_aead_generate_nonce"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_generate_nonce()</span></code></a> or
<a class="reference internal" href="../api/ops/aead.html#c.psa_aead_set_nonce" title="psa_aead_set_nonce"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_set_nonce()</span></code></a> to generate or set the nonce.</p></li>
<li><p>When decrypting, call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_set_nonce" title="psa_aead_set_nonce"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_set_nonce()</span></code></a> to set the nonce.</p></li>
</ul>
</li>
<li><p>Call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_update_ad" title="psa_aead_update_ad"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_update_ad()</span></code></a> zero or more times with fragments of the
non-encrypted additional data.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_update" title="psa_aead_update"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_update()</span></code></a> zero or more times with fragments of the plaintext
or ciphertext to encrypt or decrypt.</p></li>
<li><p>At the end of the message, call the required finishing function:</p>
<ul class="simple">
<li><p>To complete an encryption operation, call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_finish" title="psa_aead_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_finish()</span></code></a> to compute
and return authentication tag.</p></li>
<li><p>To complete a decryption operation, call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_verify" title="psa_aead_verify"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_verify()</span></code></a> to
compute the authentication tag and verify it against a reference value.</p></li>
</ul>
</li>
</ol>
<p>To abort the operation or recover from an error, call <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_abort" title="psa_aead_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_abort()</span></code></a>.</p>
<p>Having a multi-part interface to authenticated encryption raises specific issues.</p>
<p>Multi-part authenticated decryption produces partial results that are not
authenticated. Applications must not use or expose partial results of
authenticated decryption until <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_verify" title="psa_aead_verify"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_verify()</span></code></a> has returned a success
status and must destroy all partial results without revealing them if
<a class="reference internal" href="../api/ops/aead.html#c.psa_aead_verify" title="psa_aead_verify"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_verify()</span></code></a> returns a failure status. Revealing partial results, either directly or indirectly through the application’s behavior, can compromise the
confidentiality of all inputs that are encrypted with the same key.</p>
<p>For encryption, some common algorithms cannot be processed in a streaming
fashion. For SIV mode, the whole plaintext must be known before the encryption
can start; the multi-part AEAD API is not meant to be usable with SIV mode. For
CCM mode, the length of the plaintext must be known before the encryption can
start; the application can call the function <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_set_lengths" title="psa_aead_set_lengths"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_set_lengths()</span></code></a> to provide
these lengths before providing input.</p>
</div>
<div class="section" id="key-derivation">
<span id="id3"></span><h3>3.3.7. Key derivation</h3>
<p>A key derivation encodes a deterministic method to generate a finite stream of
bytes. This data stream is computed by the cryptoprocessor and extracted in
chunks. If two key derivation operations are constructed with the same
parameters, then they produce the same output.</p>
<p>A key derivation consists of two phases:</p>
<ol class="arabic simple">
<li><p>Input collection. This is sometimes known as <em>extraction</em>: the operation
“extracts” information from the inputs to generate a pseudorandom
intermediate secret value.</p></li>
<li><p>Output generation. This is sometimes known as <em>expansion</em>: the operation
“expands” the intermediate secret value to the desired output length.</p></li>
</ol>
<p>The specification defines a <a class="reference internal" href="#multi-part-operations"><span class="std std-ref">multi-part operation</span></a>
API for key derivation that allows for multiple key and non-key outputs to be
extracted from a single derivation operation object.</p>
<p>In an implementation with <a class="reference internal" href="goals.html#isolation"><span class="std std-ref">isolation</span></a>, the intermediate
state of the key derivation is not visible to the caller, and if an output of
the derivation is a non-exportable key, then this key cannot be recovered
outside the isolation boundary.</p>
<p>Applications use the <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_operation_t" title="psa_key_derivation_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_derivation_operation_t</span></code></a> type to create key
derivation operations. The operation object is used as follows:</p>
<ol class="arabic simple">
<li><p>Initialize a <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_operation_t" title="psa_key_derivation_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_derivation_operation_t</span></code></a> object to zero or to
<a class="reference internal" href="../api/ops/kdf.html#c.PSA_KEY_DERIVATION_OPERATION_INIT" title="PSA_KEY_DERIVATION_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_setup" title="psa_key_derivation_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_setup()</span></code></a> to select a key derivation algorithm.</p></li>
<li><p>Call the functions <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_input_bytes" title="psa_key_derivation_input_bytes"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_input_bytes()</span></code></a> and
<a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_input_key" title="psa_key_derivation_input_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_input_key()</span></code></a>, or <a class="reference internal" href="../api/ops/ka.html#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a> to
provide the inputs to the key derivation algorithm. Many key derivation
algorithms take multiple inputs; the <code class="docutils literal notranslate"><span class="pre">step</span></code> parameter to these functions
indicates which input is being provided. The documentation for each key
derivation algorithm describes the expected inputs for that algorithm and
in what order to pass them.</p></li>
<li><p>Optionally, call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_set_capacity" title="psa_key_derivation_set_capacity"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_set_capacity()</span></code></a> to set a limit on the
amount of data that can be output from the key derivation operation.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_output_key" title="psa_key_derivation_output_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_output_key()</span></code></a> to create a derived key, or
<a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_output_bytes" title="psa_key_derivation_output_bytes"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_output_bytes()</span></code></a> to export the derived data. These
functions can be called multiple times to read successive output from the key
derivation, until the stream is exhausted when its capacity has been reached.</p></li>
<li><p>Key derivation does not finish in the same way as other multi-part
operations. Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_abort" title="psa_key_derivation_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_abort()</span></code></a> to release the key derivation
operation memory when the object is no longer required.</p></li>
</ol>
<p>To recover from an error, call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_abort" title="psa_key_derivation_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_abort()</span></code></a> to release the key
derivation operation memory.</p>
<p>A key derivation operation cannot be rewound. Once a part of the stream has been
output, it cannot be output again. This ensures that the same part of the output
will not be used for different purposes.</p>
</div>
<div class="section" id="example-of-the-symmetric-cryptography-api">
<h3>3.3.8. Example of the symmetric cryptography API</h3>
<p>Here is an example of a use case where a master key is used to generate both a
message encryption key and an IV for the encryption, and the derived key and IV
are then used to encrypt a message.</p>
<ol class="arabic simple">
<li><p>Derive the message encryption material from the master key.</p>
<ol class="loweralpha simple">
<li><p>Initialize a <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_operation_t" title="psa_key_derivation_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_derivation_operation_t</span></code></a> object to zero or to
<a class="reference internal" href="../api/ops/kdf.html#c.PSA_KEY_DERIVATION_OPERATION_INIT" title="PSA_KEY_DERIVATION_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_setup" title="psa_key_derivation_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_setup()</span></code></a> with <a class="reference internal" href="../api/ops/kdf.html#c.PSA_ALG_HKDF" title="PSA_ALG_HKDF"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_HKDF</span></code></a> as the algorithm.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_input_key" title="psa_key_derivation_input_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_input_key()</span></code></a> with the step
<a class="reference internal" href="../api/ops/kdf.html#c.PSA_KEY_DERIVATION_INPUT_SECRET" title="PSA_KEY_DERIVATION_INPUT_SECRET"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_INPUT_SECRET</span></code></a> and the master key.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_input_bytes" title="psa_key_derivation_input_bytes"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_input_bytes()</span></code></a> with the step
<a class="reference internal" href="../api/ops/kdf.html#c.PSA_KEY_DERIVATION_INPUT_INFO" title="PSA_KEY_DERIVATION_INPUT_INFO"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_KEY_DERIVATION_INPUT_INFO</span></code></a> and a public value that uniquely
identifies the message.</p></li>
<li><p>Populate a <a class="reference internal" href="../api/keys/attributes.html#c.psa_key_attributes_t" title="psa_key_attributes_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_key_attributes_t</span></code></a> object with the derived message
encryption key’s attributes.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_output_key" title="psa_key_derivation_output_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_output_key()</span></code></a> to create the derived message key.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_output_bytes" title="psa_key_derivation_output_bytes"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_output_bytes()</span></code></a> to generate the derived IV.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/kdf.html#c.psa_key_derivation_abort" title="psa_key_derivation_abort"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_abort()</span></code></a> to release the key derivation operation
memory.</p></li>
</ol>
</li>
<li><p>Encrypt the message with the derived material.</p>
<ol class="loweralpha simple">
<li><p>Initialize a <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_operation_t" title="psa_cipher_operation_t"><code class="xref any c c-type docutils literal notranslate"><span class="pre">psa_cipher_operation_t</span></code></a> object to zero or to
<a class="reference internal" href="../api/ops/ciphers.html#c.PSA_CIPHER_OPERATION_INIT" title="PSA_CIPHER_OPERATION_INIT"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_CIPHER_OPERATION_INIT</span></code></a>.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_encrypt_setup" title="psa_cipher_encrypt_setup"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_encrypt_setup()</span></code></a> with the derived message encryption key.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_set_iv" title="psa_cipher_set_iv"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_set_iv()</span></code></a> using the derived IV retrieved above.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_update" title="psa_cipher_update"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_update()</span></code></a> one or more times to encrypt the message.</p></li>
<li><p>Call <a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_finish" title="psa_cipher_finish"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_finish()</span></code></a> at the end of the message.</p></li>
</ol>
</li>
<li><p>Call <a class="reference internal" href="../api/keys/management.html#c.psa_destroy_key" title="psa_destroy_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_destroy_key()</span></code></a> to clear the generated key.</p></li>
</ol>
</div>
</div>
<div class="section" id="asymmetric-cryptography">
<h2>3.4. Asymmetric cryptography</h2>
<p>This specification defines functions for asymmetric cryptography, including
asymmetric encryption, asymmetric signature, and two-way key agreement.</p>
<div class="section" id="asymmetric-encryption">
<h3>3.4.1. Asymmetric encryption</h3>
<p>Asymmetric encryption is provided through the functions
<a class="reference internal" href="../api/ops/pke.html#c.psa_asymmetric_encrypt" title="psa_asymmetric_encrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_asymmetric_encrypt()</span></code></a> and <a class="reference internal" href="../api/ops/pke.html#c.psa_asymmetric_decrypt" title="psa_asymmetric_decrypt"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_asymmetric_decrypt()</span></code></a>.</p>
</div>
<div class="section" id="hash-and-sign">
<h3>3.4.2. Hash-and-sign</h3>
<p>The signature and verification functions <a class="reference internal" href="../api/ops/sign.html#c.psa_sign_message" title="psa_sign_message"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_sign_message()</span></code></a> and
<a class="reference internal" href="../api/ops/sign.html#c.psa_verify_message" title="psa_verify_message"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_verify_message()</span></code></a> take a message as one of their inputs and perform a
hash-and-sign algorithm.</p>
<p>The functions <a class="reference internal" href="../api/ops/sign.html#c.psa_sign_hash" title="psa_sign_hash"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_sign_hash()</span></code></a> and <a class="reference internal" href="../api/ops/sign.html#c.psa_verify_hash" title="psa_verify_hash"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_verify_hash()</span></code></a> take a message hash as
one of their inputs. This is useful for signing pre-computed hashes, or for
implementing hash-and-sign using a <a class="reference internal" href="../api/ops/hashes.html#hash-mp"><span class="std std-ref">multi-part hash operation</span></a>
before signing the resulting hash. To determine which
hash algorithm to use, call the macro <a class="reference internal" href="../api/ops/algorithms.html#c.PSA_ALG_GET_HASH" title="PSA_ALG_GET_HASH"><code class="xref any c c-macro docutils literal notranslate"><span class="pre">PSA_ALG_GET_HASH()</span></code></a> on the
corresponding signature algorithm.</p>
<p>Some hash-and-sign algorithms add padding to the message hash before completing
the signing operation. The format of the padding that is used depends on the
algorithm used to construct the signature.</p>
</div>
<div class="section" id="key-agreement">
<h3>3.4.3. Key agreement</h3>
<p>This specification defines two functions for a Diffie-Hellman-style key
agreement where each party combines its own private key with the peer’s public
key.</p>
<p>The recommended approach is to use a
<a class="reference internal" href="#key-derivation"><span class="std std-ref">key derivation operation</span></a> with the
<a class="reference internal" href="../api/ops/ka.html#c.psa_key_derivation_key_agreement" title="psa_key_derivation_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_key_derivation_key_agreement()</span></code></a> input function, which calculates a shared
secret for the key derivation function.</p>
<p>Where an application needs direct access to the shared secret, it can call
<a class="reference internal" href="../api/ops/ka.html#c.psa_raw_key_agreement" title="psa_raw_key_agreement"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_raw_key_agreement()</span></code></a> instead. Note that in general the shared secret is not
directly suitable for use as a key because it is biased.</p>
</div>
</div>
<div class="section" id="randomness-and-key-generation">
<h2>3.5. Randomness and key generation</h2>
<p>We strongly recommended that implementations include a random generator,
consisting of a cryptographically secure pseudo-random generator (CSPRNG), which
is adequately seeded with a cryptographic-quality hardware entropy source,
commonly referred to as a true random number generator (TRNG). Constrained
implementations can omit the random generation functionality if they do not
implement any algorithm that requires randomness internally, and they do not
provide a key generation functionality. For example, a special-purpose component
for signature verification can omit this.</p>
<p>It is recommended that applications use <a class="reference internal" href="../api/keys/management.html#c.psa_generate_key" title="psa_generate_key"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_generate_key()</span></code></a>,
<a class="reference internal" href="../api/ops/ciphers.html#c.psa_cipher_generate_iv" title="psa_cipher_generate_iv"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_cipher_generate_iv()</span></code></a> or <a class="reference internal" href="../api/ops/aead.html#c.psa_aead_generate_nonce" title="psa_aead_generate_nonce"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_aead_generate_nonce()</span></code></a> to generate
suitably-formatted random data, as applicable. In addition, the API includes a
function <a class="reference internal" href="../api/ops/rng.html#c.psa_generate_random" title="psa_generate_random"><code class="xref any c c-func docutils literal notranslate"><span class="pre">psa_generate_random()</span></code></a> to generate and extract arbitrary random data.</p>
</div>
</div>


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        <div class="sphinxsidebarwrapper"><h3><a href="../index.html"><b>PSA Crypto API</b></a></h3>
IHI 0086<br/>
Non-confidential<br/>
Version 1.0.1
<span style="color: red; font-weight: bold;"></span>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../about.html">About this document</a></li>
</ul>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="intro.html">1. Introduction</a></li>
<li class="toctree-l1"><a class="reference internal" href="goals.html">2. Design goals</a></li>
<li class="toctree-l1 current"><a class="current reference internal" href="#">3. Functionality overview</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#library-management">3.1. Library management</a></li>
<li class="toctree-l2"><a class="reference internal" href="#key-management">3.2. Key management</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#key-identifiers">3.2.1. Key identifiers</a></li>
<li class="toctree-l3"><a class="reference internal" href="#key-lifetimes">3.2.2. Key lifetimes</a></li>
<li class="toctree-l3"><a class="reference internal" href="#key-policies">3.2.3. Key policies</a></li>
<li class="toctree-l3"><a class="reference internal" href="#recommendations-of-minimum-standards-for-key-management">3.2.4. Recommendations of minimum standards for key management</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#symmetric-cryptography">3.3. Symmetric cryptography</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#single-part-functions">3.3.1. Single-part Functions</a></li>
<li class="toctree-l3"><a class="reference internal" href="#multi-part-operations">3.3.2. Multi-part operations</a></li>
<li class="toctree-l3"><a class="reference internal" href="#message-digests-hashes">3.3.3. Message digests (Hashes)</a></li>
<li class="toctree-l3"><a class="reference internal" href="#message-authentication-codes-macs">3.3.4. Message authentication codes (MACs)</a></li>
<li class="toctree-l3"><a class="reference internal" href="#encryption-and-decryption">3.3.5. Encryption and decryption</a></li>
<li class="toctree-l3"><a class="reference internal" href="#authenticated-encryption-aead">3.3.6. Authenticated encryption (AEAD)</a></li>
<li class="toctree-l3"><a class="reference internal" href="#key-derivation">3.3.7. Key derivation</a></li>
<li class="toctree-l3"><a class="reference internal" href="#example-of-the-symmetric-cryptography-api">3.3.8. Example of the symmetric cryptography API</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#asymmetric-cryptography">3.4. Asymmetric cryptography</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#asymmetric-encryption">3.4.1. Asymmetric encryption</a></li>
<li class="toctree-l3"><a class="reference internal" href="#hash-and-sign">3.4.2. Hash-and-sign</a></li>
<li class="toctree-l3"><a class="reference internal" href="#key-agreement">3.4.3. Key agreement</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="#randomness-and-key-generation">3.5. Randomness and key generation</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="sample-arch.html">4. Sample architectures</a></li>
<li class="toctree-l1"><a class="reference internal" href="conventions.html">5. Library conventions</a></li>
<li class="toctree-l1"><a class="reference internal" href="implementation.html">6. Implementation considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="usage.html">7. Usage considerations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/library/index.html">8. Library management reference</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/keys/index.html">9. Key management reference</a></li>
<li class="toctree-l1"><a class="reference internal" href="../api/ops/index.html">10. Cryptographic operation reference</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../appendix/example_header.html">Example header file</a></li>
<li class="toctree-l1"><a class="reference internal" href="../appendix/specdef_values.html">Example macro implementations</a></li>
<li class="toctree-l1"><a class="reference internal" href="../appendix/history.html">Changes to the API</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../psa_c-identifiers.html">Index of API elements</a></li>
</ul>
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